what's your tilt?

over the past few days and weeks, i've been noticing the relative lack of tornadic supercells--not only over canada, but over north america.

what would cause such a dearth?

well, the ingredients are pretty much all there. moisture--no problem--dewpoints have been in the 20s in the regions of interest. instability--no problem--capes have been well over 3000 j/kg. for the most part. shear--no problem. sort of--0-6 km shears have been on the order of 40 to 50 knots. but i'll get back to this in a bit. trigger--no problem--warm fronts and cold fronts have been in abundance.

so what is it?

i think it's the tilt of the upper troughs passing through.

what are you talking about, dave?

if the trough axis is oriented nw-se, it is said to have a negative tilt. if it's oriented ne-sw, it is said to have a positive tilt. if it is aligned north-south, it has neutral tilt.

well, most of the troughs coming through on potentially explosive days have been neutrally or positively tilted.

so what does that mean for the storm environment?

well, in a nutshell, the more negative tilt the approaching trough has, the more the surface winds will back. the more the winds back, the more you increase your storm-relative low-level shear.

so put that all together, and the troughs moving through the upper atmosphere have been promoting very little backing of the low-level surface flow. so instead of being tornadic supercells, the storms are perhaps exhibiting brief supercell structures but quickly transitioning to bow echoes or other linear modes of storm.